1. Field of the Invention
This invention relates to a method of making pressed magnetic core components from low carbon steel such as "black-plate" which components exhibit low core loss and higher permeability and more particularly, it pertains to a method for reannealing and repressing low carbon steel microlaminations for use in electrical apparatus.
2. Description of the Prior Art
Microlaminations are substantially flat, elongated, rectangular particles that are formed from plain carbon steel by cutting the same into discretely-shaped particles (elongated parallelopiped of generally rectangular cross-section) following which the microlaminations are decarburized, magnetically insulated, and thereafter placed in a mold and pressed to the desired density without the use of a binder for producing the finished unitary magnetic core. U.S. Pat. Nos. 3,848,331 and 3,948,690 disclose the advantages and methods of applying such electrical insulation.
Although the microlamination concept of producing magnetic devices continuous to show great promise, the largest factors which reduce the applicability of this method of construction are the relatively high core losses and the low permeability at power frequencies. Core losses at 60 Hz are slightly higher than such losses of low carbon steel and the permeability is significantly lower. These factors, although not particularly important in magnetic structures which contain an air gap, can require significant modifications to a device having a completely closed magnetic circuit. For example, a transformer constructed from microlaminations might require an increased core cross-section or additional coil turns in order to achieve characteristics identical to those of a transformer made from punched and stacked laminations. These potential modifications can reduce the inherent economic advantage of the microlamination process.
As a result of the high level of strain in a pressed microlamination compact, greater than 90% of the 60 Hz losses are hysteretic. If this strain could be reduced, without affecting the core insulation, a significant improvement in both the core loss and the permeability could be expected.
Previous work has known that a stress relief anneal does not improve the AC magnetic performance of a pressed compact. In fact, both the core loss and the AC permeability are significantly impaired. Although this area of investigation is incomplete, it appears that the insulation breaks down at points of particle contact. This results in a significant increase in eddy currents which cause the losses to increase dramatically. Also, the permeability is reduced because of eddy current shielding and reduced flux penetration.